The invention relates to a method for operating an internal combustion engine with direct fuel injection using a pre-injection fuel amount and a main injection fuel amount.
By way of example, DE 195 19 663 A1 discloses a method for operating an internal combustion engine with spontaneous ignition, in which in a first stage a homogenous, precompressed fuel/air mix which is not suitable for spontaneous ignition is provided in the working space, and in a second stage an additional quantity of the same fuel is injected into the working space in order to bring about the spontaneous ignition. In this case, the fuel/air mix is prepared by means of external mix formation and introduced into the working space, where it is compressed to close to the spontaneous ignition point. The injection of the additional quantity of fuel in the second stage takes place in finely atomized form, avoiding contact with the walls, so as to form a mix cloud in which, on the one hand, the fuel/air ratio is no greater than the stoichiometric mixing ratio and in which, on the other hand, the spontaneous ignition is achieved.
Furthermore, DE 198 52 552 C2 discloses a method for operating a four-stroke internal combustion engine which, at part load, forms a lean base mix from air, fuel and retained exhaust gas and, at full load, forms a stoichiometric mix. At part load, compression ignition takes place, whereas at full load spark ignition takes place. Furthermore, mechanically controlled exhaust-gas retention with switchable valve closure overlap and exhaust gas throttling is provided. An activation injection can be carried out into the retained exhaust gas. The quantity of retained exhaust gas, with the valve closure overlap activated, is controlled or preset as a function of the engine speed and engine load by an exhaust-gas throttle valve which is active for all the combustion chambers. The pressure present upon opening of the intake valves is equalized for the individual combustion chambers by a cylinder-selective, cycle-consistent activation injection.
A method for operating a four-stroke, reciprocating-piston internal combustion engine is also known from DE 198 10 935 A1. It is characterized by a homogenous, lean base mix of air, fuel and retained exhaust gas and by compression ignition and direct injection of the fuel into the combustion chamber. The volume of the combustion chamber changes cyclically. The combustion chamber can be filled with fresh gas through at least one intake valve, while the combustion exhaust gases can be at least partially expelled through at least one exhaust valve. In the part-load range and in the lower full-load range, the internal combustion engine is operated with compression ignition and preferably mechanically controlled exhaust-gas retention, whereas in the full-load range and high part-load range it is operated on the spark ignition cycle.
One drawback of the methods known from the above-mentioned documents is in particular that the temperature and the composition of the working gas change in the event of changes in load. The reactivity of the mix during compression ignition is likewise altered as a result, even to the extent of causing misfires if the working gas temperatures are too low.
It is therefore the object of the invention to provide a method for operating an internal combustion engine in which the changes in the working gas temperature and composition in the event of load changes can be taken into account and/or corrected.